|Figure 1. Heat absorbed in ice-free areas of the Arctic ocean in summer is released to the atmosphere in autumn. Sea ice images from NASA/GSFC Scientific Visualization Studio; thanks to Rob Gerston (GSFC) for providing the data. Composite graphic from NOAA.|
How the loss of sea ice leads to a warmer Arctic
With reduced summer sea ice, additional solar heat is absorbed in the upper ocean, from the surface to a depth of about 65 feet (20 meters). This heat is released slowly from the ocean to the atmosphere during the following Autumn, increasing atmospheric temperatures up to about 1 mile above the surface (Figure 1).
How much warmer is the Arctic?
The red colors in Figures 2 and 3 (below) indicate areas over the Arctic where autumn near-surface air temperatures in recent years were up to 10.8 °F (6°C) warmer than those typically observed in the years prior to 2002.
From 2002 to 2005, as the extent of Arctic summer sea ice began to decline, autumn air temperatures near the surface began to rise above normal (Figure 2). As the loss of sea ice accelerated, and low or record-setting low sea ice extents were observed at the end of summer from 2007-2010 (Figure 3), autumn Arctic air temperatures near the surface rose further above normal values, and elevated temperatures were seen over larger areas of the Arctic.
|Figure 2. Anomalies for autumn in 2002-2005 represent deviations from the normal near surface air temperature values which were observed from 1968-1996. Figure from Overland and Wang1.||Figure 3. Anomalies for autumn 2007-2010 represent deviations from the normal near surface air temperature values which were observed from 1968-1996. Figure from NOAA/ESRL Physical Sciences Division.|
Rises in near surface air temperatures throughout the 21st century are projected to be especially pronounced over the Arctic Ocean during the cold season, largely driven by loss of sea ice cover.2
Heat released from the warmer Arctic changes the Arctic atmosphere
As sea ice cover is diminished during autumn, air temperature near the surface is increased, decreasing the stable stratification of the lower atmosphere.1 The result is a dome of warm air and elevated pressure surfaces over much of the Arctic ocean.
Elevated pressure surfaces over the Arctic are are indicated by red and yellow colors in Figure 4 (below) for autumn 2002-2010.
Note that the some of the largest pressure surface elevations, shown in red, occur from the East Siberian Sea to northern Alaska, a region of diminished summer sea ice cover for every year from 2002-2010.
|Figure 4. Deviations from normal autumn pressure surface elevations over the Arctic seen from 2002-2010. Figure from NOAA/ESRL Physical Sciences Division. graphic from NOAA.||Figure 5. Anomalies for autumn 2002-2010 represent deviations from normal east-west winds over the Arctic. Figure from NOAA/ESRL Physical Sciences Division.|
The warmer Arctic and changes in the Arctic atmosphere may impact the Polar Vortex
The elevated pressure surfaces above the North Pole persist into early winter, setting up conditions that tend to weaken the strong Polar Vortex winds that normally circle the Arctic in a counterclockwise direction, and may impact large scale wind patterns over the Northern Hemisphere, potentially allowing cold air to move southward.
Figure 5 (above) shows the changes in the Northern Hemisphere wind fields that are associated with late autumn surface air temperature and earlier sea loss. Blue and purple colors indicate areas with wind deviations below normal. Note the much reduced winds north of Alaska and western Canada.1
As summer Arctic open water area increases over the next decades, we anticipate there is the potential for an increasing influence of loss of summer sea ice on the atmospheric northern hemisphere general circulation in following seasons which may have impacts on northern hemisphere weather.
Figure 6. NOAA/AVHRR infrared satellite image of explosive frontal cylone generation observed by the Japanese RV Mirai in the Beaufort Sea on September 24, 2010. The ship's location is indicated in red. Figure from Inoue and Hori (2011).4
Sea ice retreat contributes to Arctic cyclone generation
The Arctic is warming faster than the rest of the globe, due to the decrease in Arctic sea ice. With less sea ice cover, the ocean absorbes more heat from the sun during summer, increasing the temperature contrast between the warm ice-free ocean and cold ice surfaces in autumn. The large temperature contrast contributes to the generation of Arctic cyclones. In the late September 2010, Japanese Research Vessel Mirai observed the explosive generation of an Arctic cyclone, shown in Figure 6.4
Scientists analyzing observations from the Mirai concluded that this is an invaluable example of the fact that sea ice retreat contributees to polar amplification of surface air temperature increase and that cyclone generation is important in the transfer of the excess heat from the ocean into the atmosphere.4
1 Overland, J.E., and M. Wang (2010): Large-scale atmospheric circulation changes associated with the recent loss of Arctic sea ice. Tellus, 62A, 1–9.
2 Serreze, M.C., A.P. Barrett, J.C. Stroeve, D.N. Kindig, and M.M. Holland (2009): The emergence of surface-based Arctic amplification. The Cryosphere, 3, 11–19.
Arctic cyclogenesis at the marginal ice zone: A contributory mechanism for the temperature amplification? Geophys. Res. Lett., doi:10.1029/2011GL047696. [PDF]